SOFC systems are well known. An SOFC typically is fueled by “reformate” gas, which is the partially oxidized effluent from a catalytic partial oxidation (CPOx) hydrocarbon reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen (H2). The CPOx reactions also release heat that serves to maintain the temperature of the reformer. A CPOx reformer is a very simple and easily controlled device with good transient behavior and dynamic range. A known disadvantage of a CPOx reformer is that it has a relatively low fuel-processing efficiency that limits overall system efficiency.
To improve stack power density and system efficiency and to reduce carbon precipitation and deposition in the system, it is known in the art to recycle a portion of the tail gas from the stack anodes through the reformer. The stack anode tail gas has a large amount of water vapor and CO2 as well as unreacted H2 and CO gases. When these gases are fed back to the reformer, endothermic “steam reforming” reactions can occur in the fuel reformer. Stack anode tail gas recycle is known to be enhanced by fuel reformer technology that can sustain its temperature in the presence of endothermic reactions. Such technology may consist of a heat exchanger construction wherein hot combustor effluent passes on one side of the heat exchanger (combustor side), and a mix of fuel, air, and recycle gas passes through the other side (reforming side). The reforming side is catalytically treated to allow for the preferred reactions to occur. This mechanization yields high fuel processing efficiencies that, in turn, yield high system efficiencies.
Disadvantages to this approach are complexity and potential durability issues with the heat exchanger/reformer device because of the higher temperatures required for endothermic reforming; the system complexity required to channel the combustor gases through the reformer; and the potential for carbon precipitation in the produced reformate which may have lower water vapor content by volume.
Where natural gas is the fuel, steam reforming with added water (no recycle) is a very common approach. In some cases, the natural gas fuel is pre-reformed to break-down higher hydrocarbons (heavier than methane) and this high-methane mix is fed directly to an SOFC stack. H2O is typically added to the reformate to allow steam reforming reactions to occur within the SOFC stack itself. This arrangement is known as “Internal Reforming” in the art. In this prior art approach, the heat required for endothermic reforming to occur is supplied by the electrochemical heat released in the SOFC stack, and not by heat exchange with the combustor gases. Internal endothermic reforming within the SOFC stack is very attractive for its high fuel processing efficiencies, but in the prior art it requires a supply of external water injection to the system.
There is a limitation, however, to the range of operation in a system with this fuel processing configuration. The system efficiency is quite high when a fraction, or all, of the fuel can be reformed internally to the stack. The problem is that the reforming process requires the stack to provide the necessary heat to support the endothermic reactions, and it is not capable of providing that heat below a certain system operating power. This means the efficiency of the system, when operating at low electric load, is that of a CPOx system and reaches the highest system efficiencies only when higher loads can support internal reforming.
What is needed in the art is a system mechanization and algorithm that incorporates the benefits of each prior art system configuration in an architecture that allows for full flexibility in fuel processing, incorporating CPOx, endothermic, and internal reforming depending upon the power load of the fuel cell system.
It is a principal object of the present invention to improve the fuel efficiency of a solid oxide fuel cell stack system over the full range of operating loads.